1. Field of the Invention
The present invention relates to footwear. The invention concerns, more particularly, a sole for footwear that includes a mechanism for adjusting stiffness characteristics of the sole.
2. Description of Background Art
Sole design for modern athletic footwear is generally characterized by a multi-layer construction that includes an outsole, midsole, and insole. The midsole typically includes a soft, foam material to attenuate impact forces and absorb energy when the footwear contacts the ground during athletic activities. Other prior art midsoles utilize fluid or gas-filled bladders of the type disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Marion F. Rudy. Although foam materials succeed in providing cushioning for the foot, foam materials also impart instability that increases in proportion to midsole thickness. For this reason, footwear design often involves a balance of cushioning and stability.
The typical motion of the foot during running proceeds as follows. First, the heel strikes the ground, followed by the ball of the foot. As the heel leaves the ground, the foot rolls forward so that the toes make contact, and finally the entire foot leaves the ground to begin another cycle. During the time that the foot is in contact with the ground, it typically rolls from the outside or lateral side to the inside or medial side, a process called pronation. That is, normally, the outside of the heel strikes first and the toes on the inside of the foot leave the ground last. While the foot is air borne and preparing for another cycle the opposite process, called supination, occurs. Pronation, the inward roll of the foot while in contact with the ground, although normal, can be a potential source of foot and leg injury, particularly if it is excessive. The use of soft cushioning materials in the midsole of running shoes, while providing protection against impact forces, can encourage instability of the sub-talar joint of the ankle, thereby contributing to the tendency for over-pronation. This instability has been cited as a contributor to “runners knee” and other athletic injuries.
Various methods for resisting excessive pronation or instability of the sub-talar joint have been proposed and incorporated into prior art athletic shoes as stability devices. In general, these devices have been fashioned by modifying conventional shoe components, such as the heel counter and midsole material, or adding a pronation control device to the midsole. Examples of these techniques are found in U.S. Pat. Nos. 4,288,929; 4,354,318; 4,255,877; 4,287,675; 4,364,188; 4,364,189; 4,297,797; 4,445,283; and 5,247,742.
Stabilization is also a factor in sports like basketball, volleyball, football, and soccer. In addition to running, an athlete may be required to perform a variety of motions including lateral movement; quickly executed direction changes, stops, and starts; movement in a backwards direction; and jumping. While making such movements, footwear instability may lead to excessive inversion or eversion of the ankle joint, a primary cause of ankle sprain. For example, an athlete may be required to perform a rapid, lateral movement on a surface with friction characteristics that prevents sliding of the sole relative to the surface. Upon contact with the surface, the lateral portion of the foot impacts the interior of the footwear causing the lateral side of the midsole to compress substantially more than the medial side. The downward incline on the interior of the footwear caused by the differential compression, in conjunction with the momentum of the athlete's body, creates a situation wherein the shoe rolls towards the lateral side, causing an ankle sprain. Similar situations which cause excessive inversion or eversion comprise one common type of injury associated with athletic activities. A shoe with high lateral (side-to-side) stability will minimize the effects of differential compression by returning to a condition of equilibrium wherein the foot is centered over the sole.
The preceding example particularly arises when footwear incorporates a midsole with cushioning qualities that do not provide sufficient stability. In order to compensate for a lack of stability, designers often incorporate devices into the upper that increase stiffness. These devices attempt to provide a stable upper to compensate for an instability in the sole. Such devices take the form of rigid members, elastic materials, or straps that add to the overall weight of the footwear, make the article of footwear cumbersome, or restrict plantar flexion and dorsi flexion. For example, U.S. Pat. No. 4,989,350 to Bunch et al. discloses an article of footwear with sheet springs attached to the ankle portion, and U.S. Pat. No. 5,152,082 to Culpepper discloses an ankle support including a plurality of stiff projections extending along the heel and ankle. U.S. Pat. No. 5,896,683 to Foxen et al. discloses a support in the form of a plurality of finger-like elements attached to the upper which does not add significant weight to the shoe and allows plantar and dorsi flexion.
U.S. Pat. Nos. 5,353,523 and 5,343,639 to Kilgore et al., which are hereby incorporated by reference, discloses an article of athletic footwear with a midsole that includes foam columns placed between rigid upper and lower plates. FIG. 1 depicts a prior art shoe 10 that includes an upper 12 which is attached to a sole 14. In addition to outsole layer 20, sole 14 includes a midsole 18 that incorporates four support elements 32. Midsole 18 also includes footframe 23, cushioning and stability component 24, midfoot wedge 40, and cushioning layer 22 which is formed from a cushioning material such as ethyl vinyl acetate or non-microcellular polyurethane foam and extends throughout at least the forefoot portion of shoe 10.
Cushioning and stability component 24 includes shell or envelope 26 having upper and lower plates 28 and 30, defining therebetween an open area of the sole, and a plurality of compliant elastomeric support elements 32 disposed in the open area. FIGS. 2 illustrate three configurations for envelope 26. In one embodiment of this prior art shoe, support elements 32 have the shape of hollow, cylindrical columns or columns containing a plurality of interior voids.
The outer surface of support elements 32 may include a plurality of spaced grooves that removably receive bands 36 and ensure uniform vertical deflection. Columns designed with straight walls that do not contain grooves have a greater tendency to buckle. Furthermore, the compliance of the columns and the overall stiffness of the midsole may be adjusted through use of bands 36 that are retained by the grooves. Generally, bands 36 that are located in a centrally located groove increase the stiffness of support element 32. By moving band 36 out of the groove and positioning band 36 near the top or bottom of support element 32, the stiffness is decreased. In this manner, the wearer may individually tune the stiffness of the midsole to his own requirements, taking into account body weight and the activity for which the shoe will be used.
Although bands 36 provide an effective method of adjusting the stiffness of support element 32, the prior art designs are difficult for a wearer to adjust. In order to have a practical effect upon stiffness, bands 36 must significantly constrict support element 32. The considerable effort that is necessary to alter the configuration of bands 36 inhibits wearers from properly adjusting the stiffness of support elements 32. Accordingly, the art requires a system for adjusting stiffness wherein a wearer may easily alter the configuration of the bands that circumscribe support elements 32.